This invention relates to the method of dissolving or etching copper and copper bearing metals with hydrogen peroxide in an acidic solution with the emphasis on, but not limited to, the etching of printed circuit boards.
Heretofore, most copper etching has been done with either acidic cupric chloride solutions, ferric chloride solutions, alkaline ammonia etchants, or acid ammonium or sodium persulfate etchants. The largest quantities of these etchants are used by the electronic and related industries in the manufacture of printed circuit boards.
While each of the above etchant product types might not be useful for every possible requirement, generally, they have been accepted by the industry because of low cost, ease of operation, or speed of etching.
Because of much stricter environmental regulations enacted by federal, state, and local governments, waste effluents from chemical operations, such as metal etching, are no longer allowed in streams, rivers, or public sewage systems unless, essentially, all dissolved chemicals have been removed beforehand. This is a costly operation and not feasible for some operations. Recycling is also difficult, if not impossible, with the above named etching types.
Acid hydrogen peroxide etchants, the subject of this invention, are relatively easy to recycle. Most of these types of etchants are based on sulfuric acid and the excess of copper can be removed as copper sulfate crystals by simply chilling the solution. The copper can be reclaimed by electrolysis. Hydrogen peroxide itself is completely non-polluting and, in fact, is used in detoxification of sewage effluents and in the treatment of drinking water in some water works instead of chlorine.
Although some acid hydrogen peroxide etchings are commercially used, such use is still limited. There are several reasons for this. Historically, the hydrogen peroxide etching process was too expensive when compared to the more widely used ferric chloride or alkaline ammonia processes, as long as there were no strict requirements on the quality of the waste effluents. However, the main reason, at this time, is that the etching speed is too slow for the larger modern operations.
This invention overcomes this obstacle of slow etching speed. Pure hydrogen peroxide is an unstable chemical. When the peroxide comes in contact with metal surfaces or metal salt solutions are added to hydrogen peroxide (including copper), the peroxide decomposes rapidly. Such decomposition would make use of the peroxide impractical in an etching solution.
To overcome the peroxide instability problem, chemicals have been found that will stabilize hydrogen peroxide to an extent that it can be handled safely for many applications. However, these stabilized hydrogen peroxide formulas are now so inert that even the etching attack on metal is reduced to an impractical level. To accelerate the etching process, additives have to be found that will accelerate the etching without destabilizing the hydrogen peroxide, i.e., such additives ideally should be effective solely on the metal/liquid interface. Several such additives have been mentioned in the literature. Most of the additives which are based on organic compounds result in a moderate increase of etch rate, but fall short when compared with processes based on ferric chloride or cupric chloride. Additives based on mercury salts or salts of noble metals (such as silver, platinum, or palladium) are good accelerators but are either too expensive or too toxic. The above listed additives also destabilize the hydrogen peroxide to some extent, making the process less economical.
The accelerator used in the practice of the invention does not have any of the above named disadvantages while it provides a five fold or more increase in etch rate when compared to the same etching solution without an additive.